Nuclear magnetic resonance imaging (“MRI”) is utilized for scanning and imaging biological tissue as a diagnostic aid, and is one of the most versatile and fastest growing modalities in medical imaging. As part of the MRI process, the subject patient is placed in an external magnetic field. This field is created by a magnet assembly, which may be closed or open. Open magnet assemblies have two spaced-apart magnet poles separated by a gap, and a working magnetic field volume located within the gap. The magnetic field produced by the magnet is applied to the subject tissue, and the resulting nuclear magnetic resonance (“NMR”) is read by a detector. The NMR data is then processed to produce an image of the tissue.
Conventionally, the elements of these imaging apparatus are sized and arranged to image an entire human body during a scan. Recently, scanning devices have been developed to facilitate imaging only a particular anatomical area of interest of the subject patient, rather than the patient's entire body. For example, such devices can be used to scan only an extremity or joint of the patient. The devices are designed such that the dimensions of the magnet gap accommodate the extremity, such as an arm, leg, or head, or a joint, such as an elbow, knee, wrist, or ankle. As used herein, the term “extremity” shall mean any such or similar portion of the human body, or a correspondingly similar portion of an animal's body.
Conventional extremity scanners, however, have a major drawback in that, due to design constraints, sufficient scanning field strength is not provided to adequately image the target body part. Typically, the usable field within the gap is provided at a strength of no greater than 0.2 tesla, which may limit some imaging applications. At least one design provides a larger field strength, but the structural design is such that weight-bearing scans are not possible, so the applications for this design are limited. In fact, most conventional designs require that an extremity to be scanned must be placed inside a small cylinder or other enclosed space. Most patients are uncomfortable and become fidgety when confined in this manner, making it more difficult to obtain meaningful diagnostic information.
There is therefore a need for a magnet structure design that has dimensions suitable for use in imaging an extremity or particular portion of a subject's body, while still providing a field strength within the magnetic field volume that will allow for clear imaging of the subject tissue. The design should provide ample room for the extremity so that the patient is comfortable. The magnet structure should also be constructed such that it can enable weight-bearing scans, providing even more diagnostic flexibility.